1. Field of the Invention
This invention relates to a method of removing Fe ions and other detrimental cations accumulated in a tinning liquid and in a water washing bath after tinning to recover and reproduce the tinning liquid and useful materials, such as a plating electric conduction assistant and tin, for reuse.
2. Description of the Related Art
Electric tinning processes are generally grouped into alkali lines and acid lines. A halogen bath, a ferro-stann-bath or the like is ordinarily used as a plating bath in acid lines. Recently, an organic acid (alkanesulfonic acid, alkanolsulfonic acid) bath with which an increase in current density and insoluble anodization can be achieved has come into use. In recent years, however, the problem of environmental pollution has attracted more attention, and regulations on disposal of waste liquids have become stricter. For this reason and because of a high price of available acid baths, a need to reduce the amount of waste liquid by forming a plating line as a closed system has arisen. Also, it is a common practice to add various expensive organic additives to a plating liquid for the purpose of improving the current efficiency and the plated appearance. Also for prevention of losses of such expensive additives in a plating liquid, recovery of the plating liquid is indispensable.
However, it has been found that as the operation of recovering the entire or a part of a waste liquid and returning the recovered liquid to a plating bath in a closed system continues, various impurities accumulate in the plating liquid and this badly influences the plating quality. In an open system such impurities are discharged out of the system along with a waste liquid. In particular, Fe ions accumulate in the tinning liquid by dissolution during predipping of a plating member to be plated (steel plate or band) in an acid tinning bath and electro-depositing metal tin to the surface of the plated member, by introduction of an acid-cleaning water-wash liquid of a pretreatment step into the plating liquid and by other causes.
Fe.sup.2+ ions in a plating liquid are changed into Fe.sup.3+ ions by air oxidation or anodic oxidation of an insoluble anode, as represented by the following formula (1): EQU Fe.sup.2+ +1/4O.sub.2 +1/2H.sub.2 O.fwdarw.Fe.sup.3+ +OH (1)
However, if Sn ions exist in the plating liquid, Fe.sup.3+ ions oxidize Sn ions by a reaction as represented by the following formula (2) EQU 2Fe.sup.3+ +Sn.sup.2+ .fwdarw.2Fe.sup.2+ +Sn.sup.4+ ( 2)
Sn.sup.4+ ions generated in this manner precipitate in a sludge (SnO.sub.2). Therefore, it can be considered that substantially no Fe.sup.3+ ions exist in the tinning liquid and that all the Fe ions are mainly Fe.sup.2+ ions. It can also be considered that the amount of Sn.sup.4+ is very small and that all the Sn ions are mainly Sn.sup.2+ ions. Thus, if Fe.sup.2+ ions exist in the tinning liquid, Sn.sup.2+ ions are oxidized to cause a sludge and the loss of precious tin is large.
According to an experiment made by the inventors of the present invention, it is known that the oxidizing velocity of Sn.sup.2+ is proportional to the square of the Fe.sup.2+ concentration. If the Fe ion concentration in a plating liquid is increased, the amount of generated sludge is sharply increased and serious problems of tin loss and of deterioration in plated surface configuration caused by contact with a contamination of the plating line and the sludge are encountered. In the case of a high-speed plating line, in particular, the oxidation of a plating liquid is promoted by involvement of air and such problems therefore become more considerable. Accordingly, a need to specially remove and control Fe ions in the plating liquid arises. Such problems are unnoticed if a low-cost bath such as a halogen bath is used and if the plating liquid is removed as a waste. In recent years, however, organic acid baths of a high unit bath cost have come into use, which are adopted for the purpose of improving the plating performance. If such baths are used, the method of renewing plating liquid by removing the plating liquid as waste is not economical. Therefore, a need for a method of reproducing a plating liquid has arisen.
If Sn ions and detrimental cations such as Fe ions coexist in a tinning liquid in any process, Sn ions are captured with priority while other detrimental ions, i.e., Fe ions and the like, cannot be removed and remain in the tinning liquid. Therefore, a method of removing Sn ions in a plating bath by some pretreatment and thereafter removing Fe ions and other detrimental cations remaining in the plating bath is generally suitable and has been accepted as a basic idea. Processes based on this method have been proposed as described below.
Japanese Patent Publication No. 57-53880 discloses a method of recovering Sn ions in a tinning liquid by using a chelate resin having a property of selectively adsorbing Sn ions and thereafter removing Fe ions and other cations by using a strong-acid cation exchange resin. In this patent, a recovered acid (the same acid as a plating electric conduction assistant) obtained by the chelate resin treatment is used to desorb Sn ions adsorbed to the chelate resin. It is therefore apparent that an amount of the acid (plating electric conduction assistant) larger than the recovered amount is required to sufficiently desorb the adsorbed ions and to promote the exchange reaction on the desorption side. The acid content in the recovered tinning liquid finally obtained is larger than the plating liquid before the Fe removing treatment because a large amount of the acid is passed through the chelate resin to stably desorb Sn ions therefrom. The recovered liquid, therefore, cannot be directly returned to the plating bath and it is necessary to remove excess acid by some means. Also, since an expensive plating electric conduction assistant must be used for Sn ion desorption, the process cost is increased. The equipment cost is increased by the provision of two necessary resin columns.
Japanese Patent Publication No. 61-17920 discloses a method of electrolytically precipitating and removing Sn ions by electrolysis of a tinning liquid and thereafter removing Fe ions and other cations by passing the remaining liquid through an H-type cation exchange resin. However, if this method is used, additives and the like in the plating liquid are anode-oxidized by electrolysis. Also, if a dragged-out liquid is processed or if the Sn ion concentration is extremely low, the electrolysis efficiency is reduced, resulting in an increase in process cost. Moreover, a troublesome operation for maintenance of the electrolytic precipitation cathode is required and Sn ions lost by electrolysis must be resupplied. Thus, this method is economically disadvantageous.
As described above, if a process in which Sn ions are selectively captured and Fe ions and other cations are thereafter removed is used, Sn ions captured by the chelate resin method or the electrolytic precipitation method cannot be efficiently recovered. Therefore, the overall operability and economy of the process are reduced by the recovery step. Moreover, excess acid is mixed in the reproduced plating liquid and Sn ions are lost by electrolysis, so that the material balance is lost. The operation of such a process cannot be repeatedly performed because the composition of the plating liquid becomes different from the original composition during repeated operation. Thus, no methods have been developed which ensure that an acid tinning bath can be continuously reproduced, and there is a demand for a process improved in economy as well as operability.